The total harmonic distortion (THD) of an electrical signal is a measurement of the amount of unwanted harmonic frequencies that are present in the electrical signal. A typical power system 10, shown in FIG. 1, includes a power source 12 generating a sinusoidal alternating current (AC) voltage 14 and current 16 at a fundamental frequency “f” (or time period “T”). A load 18 is electrically coupled to power source 12. If the impedance “Z” of load 18 is essentially a linear load with only a resistive component, voltage 14 and current 16 are generally sinusoidal, as shown in FIG. 2, and may have a phase difference among each other. However, if load 18 is non-linear the load can draw current 16 that is non-sinusoidal. For example, load 18 may be a switch-mode power supply that draws current in a non-sinusoidal manner. Because the waveform of the load current 16 is non-sinusoidal when the voltage 14 applied to the load is sinusoidal, the load is considered to be non-linear. Since the source voltage has generally a non-zero impedance, the deviation from a sine wave of current 16 will induce a distortion in voltage 14, as generally shown in FIG. 3.
As can be seen from FIG. 3, waveform distortions can drastically alter the shape of the sinusoid. The resulting complex wave is a composite of multiple harmonic waveforms called harmonics. Harmonics have frequencies that are integer multiples of the waveform's fundamental frequency “f.” For example, given a 60 Hz fundamental waveform, the second, third, fourth and fifth harmonic components will be at 120 Hz, 180 Hz, 240 Hz and 300 Hz respectively. Thus, harmonic distortion is the degree to which a waveform deviates from its pure sinusoidal values as a result of the summation of all these harmonic elements. In contrast, an ideal sine wave has zero harmonic components, with no distortion of the sine wave.
Total harmonic distortion measurements may be utilized to characterize the power quality of electric power systems. THD is generally defined by Equation 1, below:
                              THD          f                =                                                            V                2                2                            +                              V                3                2                            +                              V                4                2                            +              …              +                              V                n                2                            +              …                                            V            1                                              Equation        ⁢                                  ⁢        1            where Vn is the root-mean-square (RMS) voltage (V) of the nth harmonic and n=1 is the fundamental frequency (f). Stated another way, THD is the ratio of the square root of the sum of the squares of the harmonic components to the root mean square of the component at the fundamental frequency.
Harmonic frequencies in the power grid are a frequent cause of power quality problems. Harmonics in power systems can result in increased heating in equipment and conductors that are coupled to the power system. In addition, harmonics can cause misfiring in variable speed drives and torque pulsations in rotating electrical machinery. In power systems, lower THD results in a reduction in peak currents, heating, emissions, and losses. Reduction of harmonics in power systems is thus desirable.